A supremely high and perfectly muscular ass on what would otherwise be a total nerd, thereby doubling the sex appeal factor for women. In other men, the nerdbutt demands a solid on the spot sucker punch.

Jen: "Ugh! All Drew does is play WoW and design jersey logos for high school frisbee teams... But he has the greatest ass I've ever seen."
Gabe: "Nerdbutt!"